How can I change a default submit button in a sidebox to an image button?

Hello, In my MailChimp sidebox I am trying to edit the default submit button code to use an image instead of the default grey "submit" button.

The current code: ```
$content .= '<input type="submit" name="submit" value="'Subscribe now" />';

All I want to do it change it to be an image from my templates image folder: Button_Submit.png
I can reference the image correctly using the code here:

debug.print zen_image($template->get_template_dir(HEADER_LOGO_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . 'cButton_Submit.png')

However I am having a hard time getting the image to display as the submit button. I am trying to use: ```
$content .= '<input type="submit" width="300" height="200" name="submit" style="background-image: url(../../../../images/icons/Button_Subscribe.png);" value="Subscribe Now" />'; 

and it works.. kinda. But instead of replacing the button it just adds the image to the middle of that ugly default grey button. How can I make it actually replace the old button?
Thanks,
Jacob

To use only an image, instead of a normal submit button:
Change <input type="submit"to```
<input type="image"

Thanks!